What Is a Gene Mutation?
Gene Mutation: A gene mutation is a change in the nucleotide sequence of DNA, which may alter the instructions for building proteins or regulating cellular processes.
Mutations are the ultimate source of all genetic variation. Without them, every individual in a population would carry identical genetic information, leaving no raw material for natural selection or evolution.
Mutations can arise in two main ways:
- Spontaneously , through errors in DNA replication or repair
- Induced , through exposure to external agents called mutagens
The consequences of a mutation depend on:
- Where in the genome it occurs (coding vs. non-coding region)
- Which cell type is affected (germ cell vs. somatic cell)
- What change the mutation causes at the protein level
Base Substitutions: One Letter Changed
Base Substitution: A base substitution is a gene mutation where one nucleotide in the DNA sequence is replaced by another nucleotide.
Think of it like a typo in a sentence: "The dog ate the pie" becomes "The dog ate the pig." The structure is mostly intact, but the meaning can change , sometimes dramatically, sometimes not at all.
Base substitutions are the most common type of point mutation. They fall into three functional categories depending on their effect on the resulting protein:
| Type | Effect on Codon | Effect on Protein |
|---|---|---|
| Same-sense (silent) | Different codon, same amino acid | No change |
| Mis-sense | Different codon, different amino acid | Amino acid changed |
| Nonsense | Codon becomes a stop codon | Protein truncated |
The degeneracy of the genetic code (multiple codons coding for the same amino acid) acts as a buffer , many base substitutions produce same-sense mutations with zero effect on the protein. This is evolution's built-in error tolerance.